A radio wave is generally defined by field intensity and polarization. However, a radio wave actually received by an antenna is generally evaluated only by field intensity. Thus, polarization is only considered in special cases such as a communication using an antenna taking advantage of a circular polarization characteristic.
The reason that an evaluation of polarization is not considered is that propagation of radio waves between the transmitting and receiving sides in a communication over a relatively wide range like broadcasting is very complicated due to reflections, shielding or scattering caused by structures, and therefore it is considered more effective to adopt a stochastic approach assuming that received polarized waves are random.
In this case, when an antenna characteristic used for equipment or a propagation characteristic of radio waves emitted from an antenna is measured, it is only an evaluation based on emission power or field intensity that is required. For example, directive gain of an antenna is generally measured based on electric power and a polarization characteristic is not considered except special cases.
For measurement of field intensity of an incoming signal when a communication service area is decided, an evaluation is also generally based on only electric power for similar reasons.
However, as the application range of radio communication expands, there is a tendency to reduce one radio communication area to improve the utilization rate of radio waves or to increase the amount of information transmitted. When the distance between the transmitting side and receiving side is shortened, radio waves are also propagated under simple conditions, and therefore the precondition that the polarization characteristic is random can no longer be held. For this reason, for measurement of such a radio wave environment, not only electric power but also the polarization characteristic needs to be taken into consideration.
Furthermore, the same applies to an evaluation of antenna characteristics. Especially when an antenna is used in close contact with the human body as in the case of a cellular phone, even if the antenna is the one that emits linear polarization as in the case of a monopole antenna, when the antenna is actually used the effect of the human body close to the antenna on the polarization characteristic is also so great that the polarization characteristic of the antenna may not be linear polarization, either.
However, since a conventional apparatus needs to detect phase information of radio waves at a carrier signal frequency to measure the polarization characteristic of radio waves, the problem is that it requires a very complicated structure.